Fuel pump - current

The purpose of this test is to measure the current draw of the fuel pump to determine efficient operation while making sure that there are uniform current pulses for each section of the commutator.

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

  1. Locate and remove the fuel pump fuse.
  2. Insert the fuse extension and fuse.
  3. Connect the low amp current clamp to PicoScope Channel A.
  4. Select the 60A range and zero the current clamp before connecting onto fuse extension,
  5. Note the current direction to avoid inverting your waveform.
  6. Minimise the help page and with the example waveform on screen PicoScope has already selected suitable scales for you to capture a waveform.
  7. Select GO or press the spacebar to see live data.
  8. Turn the Ignition to ON, engine OFF.
  9. Allow the pump to run then stop.
  10. When the pump has stopped, start and run the engine for about 30 s.
  11. With your live waveform on screen select STOP or press the space bar to stop your capture.
  12. Turn off the engine.
  13. Use the WAVEFORM BUFFER and ZOOM tools to examine your waveform.

Example waveform

Waveform notes

  • Peak ‘in rush current’ when fuel pump first activated from ignition on
  • Using rulers measure peak measurement of current draw from pump
  • Trigger position as indicated by Yellow diamond
  • Measurement added for maximum current during activation. Can use rulers but just showing the different options available.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select Fuel pump current.

Example fuel pump

Further guidance

Refer to vehicle technical data for specific test conditions and results.

The waveform in Figure 2 shows the activation of the fuel pump priming the low pressure fuel circuit and the starting of the engine. From this we can measure the intial in-rush of current to get the fuel pump running. In the example we measured 19.03 A. This high-current draw is to be expected as the pump begins to turn. Once it is up-and-running the current drops to around 8A.

If you zoom in on the waveform you can see the pump waveform in even more detail. It shows a current pulse for each sector of the commutator. The majority of fuel pumps have 6 to 8 sectors. Our example has 8, but if you are unsure you can count the pulses quickly and easily in PicoScope 6.

By using the vertical rulers you can mark the waveform where the pattern appears to be repeated.
Align the two rulers at this point and apply the Falling Edge Measurement. This will reveal the number of sectors present on the commutator.
With the rulers aligned you can also use the Frequency and RPM indicator which will indicate the speed of the fuel pump. In this example, the measured speed is 8190 RPM.

A known good fuel pump waveform will generally have a seesaw pattern with relative consistency and minimal variation between the highs and lows. This can make it difficult to find the number of sectors on the commutator. A bad waveform will show large or irregular drops in the pattern, with large differences between the highs and lows. These obvious inconsistencies can help identify a worn spot on the commutator or a short in the armature.

When you have a fuel pump issue you will experience loss of power. The pump may still pressurize the system and the engine may even start, but when the demand from the fuel pump is high, there will be a distinct lack of power. This can be down to a number of issues: there could be a blockage in the fuel lines, the fuel filter could be blocked or the non-return valve could no longer be operating correctly. A repeated feature on the waveform can indicate wear and an impending failure.

Time rulers used to mark sections on the commutator.
Frequency and RPM indicator measurement taken between the rulers
Reference waveform from known good fuel pump

Our waveform in Figure 4 shows a very uneven current draw on a number of sections when compared to a known good reference waveform from the waveform library. (Click here for more information regarding reference waveforms.)

This issue could be down to a poor conducting commutator or worn bushes. We would normally expect a current draw from this type of pump to be around 2.5 A, but the above waveform is above 0.5 A. From what we can see there is enough evidence to warrant removing the pump for further investigation. 

GT032-4

Disclaimer
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • 20 A / 60 A DC (low amps) current clamp

    £111.00

  • Fuse extension leads kit

    £89.00

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1 comment | Add comment

Martin Rubenstein
September 11 2017

I had to apply low-pass filtering (set at 80Hz) in order to see the current waveform because there was so much noise it was impossible to see otherwise.  I will send Steve Smith the before-and-after waveforms.

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Guided test: Fuel pump - current